Male and female brains are different. The overarching goal of the proposed studies is to understand the molecular mechanisms of sex differences in the brain, focusing on the substantia nigra (SN) as a model. It is well established that testosterone, secreted by the testes, before or after birth, acts on the male brain to masculinize specific neural networks, which results in specific masculine behaviors. The classical view is that gonadal androgens are the only factors involved in the masculinization of the brain. Discovery of differential gene expression in the mammalian brain before the gonads start producing androgens led us to propose an alternative hypothesis. We propose to explore that sex differences in brain function and behavior may be caused in part by genetic factors not produced by the gonads. We have demonstrated specific expression of the Y-linked, testis-determining factor Sry, in two regions of the adult brain: the substantia nigra (SN) and the mammillary bodies. Reduction of Sry expression in the SN resulted in a strong decrease of tyrosine hydroxylase as well as in sensorimotor impairment. In order to understand the role of Sry in sex differences in SN function, we will investigate the expression profile of Sry in the adult brain, identify the cell types in which Sry is expressed, measure the level of expression of Sry and test whether it correlates with TH expression (Specific Aim 1). We will explore the consequences of down-regulating and upregulating Sry in the SN on TH-positive neurons and the sensorimotor behaviors they control, and whether Sry action in males compensates for the lack (or lower levels) of a female-specific factor. Specifically, we will test if this female factor could be estradiol, or dosage of the X chromosome (Specific Aim 2). Finally, we will elucidate the molecular mechanisms of action of Sry on dopaminergic neurons using NT2N cells as a model of dopaminergic neurons (Specific Aim 3). This proposal intends to improve our general understanding of sex differences in brain function. The characterization of the molecular mechanisms by which male and female brains differ impacts our understanding of sex differences in psychiatric and neurological disorders such as Parkinson's disease, attention deficit disorder, or depression.